Google Now has become a prominent feature for most Android users. The voice assistant provides an easy platform to stay involved with everything that is going on in the world and perform things you enjoy as well. However, Google’s voice assistant is not yet able to interact with all apps due to the absence of specific APIs.
Right now, it is not possible to send messages on WhatsApp via Google Now due to the lack of API. Nevertheless, the root access can solve this problem in its best possible way. With Tasker and Autovoice app (and of course WhatsApp), you can send messages on WhatsApp using your voice commands with the help of Google Now.
If you want to try this, look at this XDA thread. You can manually create the Tasker profile or download the profile (in xml file formats) from the thread and import it to Tasker using the profile import option (Home -> Profile -> Import).
Though, it does not very difficult to do the thing, your phone needs root access and you should be familiar with Tasker and Autovice apps. Tasker is a very powerful Android smartphone automation application, while Autovoice is a plug-in for Tasker that allows you to add your own custom commands to Google Now.